It is known that the air filters that are used in the apparatuses where an air filtration is required, are much more efficient if the material forming the filters contain electric charges.
Such air filters hereinafter called &lt;&lt;electrostatic filters&gt;&gt; are indeed capable of catching a greater amount of dust than the conventional filters which do not have electric charges. This difference comes from the fact that attractive forces are exerted between the material forming the filters and the dust which, in a natural manner, also contains electric charges.
There are presently three methods for obtaining such electrostatic filters.
The first one consists in producing a permanent lack of balance at the molecular level, between the electric charges of a given material. The product obtained by this method is usually called a &lt;&lt;electret&gt;&gt;.
The second method consists in grafting ions, usually by bombardment, in the material forming the filter.
The third method uses the principle of triboelectricity, whereby electric charges are generated when at least two different materials are rubbered together. The product obtained by this third method are usually called &lt;&lt;triboelectric filters&gt;&gt;.
By way of example of reduction to practice of this third method, reference can be made to British patent No. 2,190,689 issued in 1989 in the name of National Research Development Corporation, which discloses a triboelectric air filter consisting of a mixture of (1) polyolefin fibers such as polyethylene, polypropylene or ethylene and propylene copolymers, with (2) fibers of another polymer containing hydrocarbon functions substituted by halogen atoms, such as chlorine. In this British patent, the weight ratio of fibers (2) to fibers (1) ranges between 70:30 and 20:80 and preferably between 60:40 and 30:70.
By way of other examples of reduction to practice of this third method, reference can also be made to U.S. Pat. No. 5,470,485 issued in 1995 in the name of FIRMA CARL FREUDENBERG, which discloses a triboelectric air filter consisting of a mixture of (1) polyolefin fibers with (2) polyacrylonitrile fibers. Once again, the weight ratio of fibers (2) to fibers (1) ranges between 70:30 and 20:80.
In all cases, several important characteristics distinguish the electrostatic filters from each other. These characteristics include the amount of electric charges, the intensity of the generated electrostatic fields, and time duration of the electrical fields. To be efficient, a filter must ideally have a high number of charges which produce intensive electrostatic fields for a period of time at least equal to the life time of the filter.
All these characteristics depend on the method used for producing the charges, from the selection and proportions of the constituting materials of the filters and finally from the conditions of use of the filter. The conditions of use are, due to the existence of a standard method for measuring the efficiency of the filters, implicitly considered as constant. Therefore, this distinction is essentially established at the level of the method of manufacture and the selection of the materials forming the filter.